The present invention relates primarily to an improved VAD (Vapor-phase Axial Deposition) apparatus used for producing porous preforms for optical fiber production. The invention may be applied as well, however, to a so-called "outside vapor-phase oxidation" method, the basic principles of which are the same as employed with the VAD apparatus.
A conventional VAD apparatus is depicted schematically in FIG. 1. In FIG. 1, reference numeral 3 indicates an oxyhydrogen burner employing a mixture of oxygen and hydrogen as a combustion gas. The burner 3 also includes passages through which gases such as SiCl.sub.4, GeCl.sub.4, Ar, etc., are passed to the flame 4. Consequently, there are produced fine particles of SiO.sub.2, GeO.sub.2, etc. the fine particles of SiO.sub.2 or GeO.sub.2 are deposited on a target member 2, which may be a quartz bar, to grow a porous preform 1 thereon. Reference numeral 5 indicates a wall (sometimes called a "muffle") which defines a deposition chamber around the preform 1. The wall 5 can be made of ordinary glass, quartz glass, or the like. A discharge hole 6 is formed in one side of wall 5, and the stem of the target member 2 extends upwardly through another passage formed at the upper end of the chamber.
The conventional apparatus of FIG. 1 suffers a serious drawback. That is, because the wall 5 is subject to electrostatic charging and the particles themselves are charged, clusters of particles tend to collect on the wall 5. Some of these clusters near the top of the chamber, when they become sufficiently large, drop off the wall 5 and strike the growing preform 1 causing nonuniformities therein. Others of the following clusters may fall into the flame 4, resulting in nonuniform scattering and again nonuniformities in the growing porous preform 1. Such a preform may be unsuitable for use in the production of optical fibers.